The present invention relates to measurement of a thickness and a thickness variation (its degree and direction) of a coating formed on a linear body.
It is very difficult to use an optical fiber itself as a light transmission medium due to problems originating from its material. Therefore, in order to maintain initial strength (immediately after manufacture) of optical fibers and to assure their long-range durability, it is now a general procedure to coat an optical fiber with resin immediately after the wiredrawing to obtain a coated optical fiber.
FIG. 36 illustrates this procedure. An optical fiber 3 is formed by wiredrawing an optical fiber material 1 while heating and melting its tip portion by a furnace 2. As a general procedure, the optical fiber 3 is sequentially passed through a first pressurizing die 4A, first curing furnace 5A, second pressurizing die 4B and second curing furnace 5B to become a coated optical fiber 6 having two resin coating layers on its outer surface, which is then wound up on a reel 6 via a capstan 7. Examples of resin coating materials for the coated optical fiber 8 are polymers including thermosetting resins such as a silicone resin, urethane resin and epoxy resin, ultraviolet-curing resins such as an epoxy-acrylate, urethane-acrylate and polyester-acrylate, and radiation-curing resins.
In order to improve transmission characteristics and mechanical characteristics of the coated optical fiber 6, it is important that the resin coating be formed around the optical fiber 1 in a concentric manner.
On the other hand, when a wiredrawing speed is increased to improve productivity of optical fibers, it is likely that the thickness of the resin coating will vary, probably because a temperature increase in the optical fiber 1 causes nonuniformity of resin flow in the pressurizing dies 4A and 4B. The thickness variation also occurs when some dust is mixed into the resin.
Therefore, it is necessary in a wiredrawing manufacturing line of optical fibers that the thickness variation of the optical fiber 6 be measured within the manufacturing line and, upon occurrence of the thickness variation, control be properly performed to reduce the drawing speed or to stop the wiredrawing process.
Referring to FIG. 37, an example of a conventional thickness variation measuring method is described below, which is disclosed in Japanese Patent Application Unexamined Publication No. Sho. 60-238737. As shown in the figure, the thickness variation is measured by irradiating a side face of a coated optical fiber 10 being wiredrawn with a laser beam 12 emitted from a laser light source 11 and detecting a forward scattering light pattern 13.
FIG. 38 illustrates the principle of this measuring method. To simplify a discussion, it is assumed that the coated optical fiber 10 consists of a glass portion 10a and a resin portion 10b. Due to a difference in the refractive indices of the two portions (usually, the refractive index n.sub.g of the glass portion 10.sub.a is approximately equal to 1.46, and the refractive index n.sub.r of the resin portion 10b is in the range of 1.48 to 1.51), the forward scattering light pattern 13 includes a central light flux 13a that has passed through the resin portion 10b, glass portion 10a and again through the resin portion 10b, and peripheral light fluxes 13b that have been transmitted through only the resin portion 10b. Therefore, the thickness variation can be detected based on the degree of symmetry in the horizontal direction in FIG. 37 and a ratio between light powers detected on both sides of the forward scattering pattern 13.
However, the above thickness variation measuring method is available only in such cases that the light passing through both of the resin portions 10b and the glass portion 10a and the light passing through only the resin portion 10b can be discriminated from each other clearly on both sides of the forward scattering pattern 13. For example, the thickness variation cannot be detected properly in the following cases: the coating diameter is small and the resin portion 10b is thin (FIG. 39) and the thickness variation is too large (FIG. 40). In the case of FIG. 39, since the resin portion 10b is too thin, there exists no light that passes though only the resin portion 10b, that is, all the light passes through both of the resin portions 10b and the glass portion 10a, so that the thickness variation cannot be detected. In the case of FIG. 40, since the resin portion 10b is very thin in the lower portion (as seen in FIG. 40), there is no light that passes through only the lower resin portion 10b. Therefore, although occurrence of the thickness variation will be known, its degree cannot be detected.